The present invention relates to a correlated superconductor single flux quantum analog-to-digital (A/D) converter, and in particular to a correlated single flux quantum A/D converter that quantizes an analog signal by multiple Josephson junctions of direct current (DC) superconducting quantum interference device (SQUID) quantizers.
There is a need for high performance and low power A/D converters to digitize high frequency analog signals for high speed signal processing applications such as in radar, communications, and sensor systems. Superconductor A/D converters have shown a potential to achieve superior performance at much lower power than many conventional integrated circuit-based A/D converters. At the present time, existing superconductor A/D converters are able to achieve a sampling speed of 10-25 million samples per second (MSps) with a resolution of 12-14 bits per sample. To achieve this performance range, however, the conventional circuit architecture of a superconductor A/D converter pushes the available superconductor junction and integrated circuit processing technology to the limit, especially for niobium (Nb), niobium nitride (NbN), and yttrium-barium-copper oxide (YBCO) superconductor integrated circuits. Exceeding the performance level of 25 MSps sampling speed and 14-16 bits per sample resolution is generally regarded to require extensive, very high speed digital signal processing which may not be feasible.
Many different A/D converter concepts have been proposed for superconductor implementation. These can be classified as:
(1) Fully parallel A/D converters which use 2.sup.N Josephson junctions as threshold detectors for N bits. These junctions are connected to the analog signal by an analog divider such as an R-2R ladder. PA0 (2) Analog folder A/D converters which use N SQUIDs to perform Gray code quantization to N bits. The N SQUIDs are connected to the analog signal by a binary divider network. PA0 (3) Single flux quantum tracking A/D converters which use a single SQUID to establish all 2N threshold levels of an N-bit A/D converter. At each threshold crossing of the SQUID. the SQUID generates a single flux quantum (SFQ) pulse. A single flux quantum counter accumulates the SFQ pulses, which tracks the analog signal. PA0 (4) Voltage-controlled oscillator--counter A/D converters which measure the frequency of a voltage controlled oscillator. The analog signal is connected to the voltage-controlled oscillator (VCO) whose frequency is controlled by the analog voltage.
An oscillator--counter superconductor A/D converter has a generic circuit diagram shown in FIG. 1. The circuit includes a SQUID quantizer 2 which comprises an inductor 4 and two Josephson junctions 6 and 8 in a symmetrically arranged loop. The Josephson junctions 6 and 8 each have an equivalent circuit shown in FIG. 1a, with a junction resistance R.sub.J and capacitance C connected in parallel with an ideal Josephson junction. Returning to FIG. 1, a current signal 10 is summed with a DC current supply 12, and the DC-biased current signal flows through a resistor R connected in parallel with the SQUID quantizer 2 to produce a voltage V across the SQUID quantizer. In response to the Voltage V, the Josephson junction 8 generates a series of SFQ pulses 14 having a pulse frequency that is directly proportional to V, with the relationship
V=.PHI..sub.o f PA1 F.sub.max .gtoreq.2.sup.N F.sub.s PA1 (a) a superconducting quantum interference device (SQUID) quantizer having at least one pair of Josephson junctions, the SQUID quantizer being connected to an input analog signal to coherently produce SFQ pulses from the analog signal by both Josephson junctions; PA1 (b) at least one pair of coherently clocked single flux quantum aperture pulse gates connected to the respective pair of Josephson junctions to transmit pulses that represent quantized samples of the analog signal; PA1 (c) at least one pair of single flux quantum flip-flops connected to the respective pair of pulse aperture gates, the flip-flops each producing a count of the pulses from the respective junction and pulse aperture gates; and PA1 (d) at least one asynchronous SFQ pulse combiner connected to combine the counts from the pair of flip-flops within a precise sampling interval, the combined counts forming an output SFQ pulse stream that represents the more significant bits of the analog signal. PA1 (e) an SFQ flip-flop counter which encodes the SFQ pulse stream from the asynchronous SFQ pulse combiner. PA1 (a) a plurality of Josephson junctions in a superconducting quantum interference device (SQUID) quantizer; PA1 (b) a plurality of pairs of coherently clocked single flux quantum pulse aperture gates connected to their respective pairs of Josephson junctions of the SQUID quantizers of the final row which generate pulses that represent quantized samples of the analog signal; PA1 (c) a first row of a plurality of pairs of single flux quantum flip-flops connected to the respective pairs of pulse aperture gates, the flip-flops each producing a count of the pulses from the respective pulse aperture gate; PA1 (d) a first row of a plurality of asynchronous SFQ pulse combiners each connected to combine the SFQ pulses transmitted by two flip-flops of the first row within a precise sampling interval; PA1 (e) at least one successive row of additional single flux quantum flip-flops connected to the asynchronous SFQ pulse combiners of a preceding row; PA1 (f) at least one successive row of additional asynchronous SFQ pulse combiners connected to combine the counts from two flip-flops of a preceding row; and PA1 (g) a final single flux quantum flip-flop counter connected to count the pulses from the preceding row of flip-flops to generate the output digital data that represents the analog signal. PA1 (a) a plurality of Josephson junctions connected in parallel with each other in a SQUID, each SQUID having a signal terminal connected to receive the input analog signal and a ground terminal; PA1 (b) a plurality of coherently clocked pulse aperture gates having a plurality of inputs connected to the respective signal terminals of the Josephson junctions and a plurality of outputs to transmit pulses that represent quantized samples of the analog signal; PA1 (c) a first row of a plurality of single flux quantum flip-flops having a plurality of inputs connected to the respective pulse aperture gates, the flip-flops each having a carry to transmit the scaled pulse rate and a read-out to produce a count of the pulses from the respective pulse aperture gate; PA1 (d) a first parallel-serial converter shift register having a plurality of inputs connected to the respective read-outs of a first row of flip-flops to convert the data from the flip-flops to a first set of serial data; PA1 (e) at least one successive row of additional single flux quantum flip-flops each having two inputs connected to the outputs of two flip-flops of a preceding row, and a carry and a read-out to produce a combined count of pulses from the flip-flops of the preceding row; PA1 (f) at least one additional parallel-serial converter shift register having a plurality of inputs connected to the respective read-outs of a preceding row of flip-flops to convert the data from the flip-flops to an additional set of serial data; PA1 (g) an asynchronous SFQ pulse combiner connected to combine the counts of pulses from the preceding row of flip-flops to generate a final set of serial data; and PA1 (h) a plurality of single flux quantum flip-flops connected in series, including:
wherein .PHI..sub.o is a constant defined by h/2e, where h is Planck's constant and e is the magnitude of the charge of an electron. .PHI..sub.o is approximately equal to 2.times.10.sup.-15 V.cndot.s. The pulses 14 are transmitted to an aperture gate 16, which is clocked to transmit or interrupt the SFQ pulses according to the sampling rate F.sub.s. The aperture gate is connected to a single flux quantum counter 18, usually a chain of flip-flops, which operate asynchronously to accumulate the total number of pulses generated by the oscillator 2 and transmitted by the aperture gate 16 during each sampling interval. At the end of each sampling interval, the counter is "read-out" as an N-bit binary number and the counter is "reset" for the next sampling interval. The aperture gate and the counter read-out generally use synchronized clock signals from a common clock.
The performance of this generic superconductor A/D converter described above is limited in resolution N and sampling rate F.sub.s by
wherein F.sub.max is the maximum SFQ frequency of the junction 8, N is the binary bit resolution, and F.sub.s is the sampling speed. Since N represents the number of bits per sample and the output data is in a binary form, 2.sup.N represents the number of quantization levels per sample. The sampling rate F.sub.s must be more than twice the highest frequency of the analog signal bandwidth to be sampled according to the Nyquist Sampling Theorem. The SFQ frequency of the oscillator-quantizer 8 must be at least the number of quantization levels per sample times the sampling rate, that is, 2.sup.N F.sub.s to produce a digital output signal that adequately represents the input analog signal.
There is presently a need for a high performance and low power A/D converter at 20 MSps sampling rate and 20 bits per sample resolution for radar applications. Another application calls for an A/D converter with a sampling speed of 150 MSps and a resolution of 16 bits per sample, and yet other applications call for a sampling speed of 20 GHz with a resolution of 10 bits per sample. According to the above relation, all of these applications require Fmax.apprxeq.10.sup.13 Hz. Some applications in space and cryogenic sensor systems require low power consumption thereby necessitating the use of superconductor A/D converters.
Therefore, there is a need for a superconductor A/D converter that is able to achieve the performance of Fmax.gtoreq.10.sup.13 Hz, using existing superconductor integrated circuit processing technology.
Furthermore, a conventional superconductor oscillator-counter A/D converter has a single flux quantum binary ripple counter connected to only one of the two Josephson junctions of a DC SQUID quantizer. A fixed current is applied to the gate of the SQUID to set the SQUID in a voltage state. A low resistance shunt may be required to reduce thermally induced frequency fluctuations which set an intrinsic noise floor and limits corresponding size of the least significant bit (LSB), and to achieve a high linearity of the pulse frequency generated in response to the analog input signal. As described above, the pulse frequency of a Josephson junction is theoretically perfectly linear with the applied voltage with a relationship given by V=.PHI..sub.o f. However, because of the non-linear current vs. voltage (I/V) characteristics of the basic DC SQUID, the frequency may not be sufficiently linear with respect to the analog input current, especially when the current is small, for example, when the current is near the junction critical current, which is the minimum current for activating the converter circuit.
Therefore, there is a further need for a superconductor A/D converter that is able to achieve an arbitrarily high linearity in converting an analog input signal to the frequency count of the single flux quantum oscillator.
Additionally, the small shunt resistor R&lt;&lt;R.sub.J (FIG. 1) reduces the sensitivity of the A/D converter to the analog signal current. Therefore there is a need to increase the sensitivity of the oscillator-counter A/D converter.